1. Field of the Invention
This invention relates to casing seals for wellheads, and in particular to a metal seal for sealing between the casing and the wellhead bore.
2. Description of the Prior Art
In a completed oil or gas well, one or more strings of casing is cemented in the well. A wellhead housing is located at the surface for supporting the upper end of the casing. The wellhead housing includes a lower portion through which the casing protrudes upwardly. A casing hanger supports the casing in the lower wellhead housing. After cementing, the upper end of the casing is cut off a selected distance above the lower wellhead housing.
An upper wellhead housing is bolted to the upper end of the lower wellhead housing. The upper wellhead housing has a bore that receives the upper end of the casing. A casing seal seals the upper end of the casing to the bore of the upper wellhead housing to prevent leakage from the annulus between the casing and the upper wellhead housing.
Frequently, the axis of the casing will be slightly off center relative to the axis of the bore of the upper wellhead housing. Because the casing is cemented in the well, it cannot be moved to change the axial alignment. With elastomeric seals, slight misalignment presents no great problem. Many wells, now, however, have metal seals. Metal seals are longer lasting and are not subject to deterioration from certain well fluids to the extent that elastomeric seals may be. However, a metal seal requires a very precise fit in order to accomplish sealing.
In the prior art technique the upper end of the casing is machined at the well site until it is coaxial with the bore of the upper wellhead housing. The metal seal is then located in place. While this technique works, it is expensive and time-consuming to machine the casing at the field site.